The invention relates to steering wheels, particularly for motor vehicles and, more particularly relates to a lightened steering wheel, particularly for this type of use, produced from a stampable, reinforced, synthetic material, and a process for the manufacture of such a steering wheel.
Steering wheels, particularly of motor vehicles, have to meet conflicting requirements.
For example, they have to be relatively light in order to have a reduced mass and a small moment of inertia with respect to the axis of the steering column for reasons particularly of comfort. The small mass means that, when the vehicle is moving at high speed or on carriageways which are in poor condition, the steering wheel does not vibrate. The small moment of inertia facilitates manoeuvring steering locks and changes in direction when the vehicle is used.
Moreover, for reasons of safety, the steering wheel has to be capable of withstanding considerable deformations resulting from stresses in the event of violent impact, particularly from the front. In particular, if the driver's thorax comes into contact with the steering wheel the latter must not give rise to contusion injuries or cause wounds by cutting or piercing.
It will therefore be seen that it is not a simple matter to manufacture a motor vehicle steering wheel which satisfies all these constraints and which also has a relatively modest cost price.
As well as these technical constraints, aesthetic requirements should be added since the steering wheel must have an attractive appearance and a pleasant feel when touched.
According to the prior art, various solutions have already been proposed for constructing steering wheels which meet most of these requirements.
A steering wheel comprises, essentially, a central hub and a peripheral rim which are connected together by at least one approximately radial branch or arm.
Excluding the hub, which is generally made of metal, such steering wheels generally comprise other metallic components. The current trend is to use such a metallic hub and to combine it with a rim and branches with metallic reinforcements or frames in order to meet both mechanical and safety constraints.
As set forth in the French Patent Application No. 2,518,479, use is made of a metallic hub with which branches and a rim are combined, which are obtained by means of a double injection moulding technique during which firstly the outer part made of a plastic material is manufactured, which acts as a shell into which another, charged plastic material, acting as a core, is then injected.
According to another technique which is the subject of European Patent Application No. 0,139,015, a steering wheel is manufactured with metallic branches and hub with which a rim made of a plastic material charged with continuous fibres is combined.
According to another technique described in European Patent Application No. 0,173,826, use is made, for the branches and the rim, of continuous, long fibres embedded in a resin which is wound and with which slotted metallic sleeves, allowing a suitable winding of the fibres whilst avoiding cleavage of their various layers in the zones where they diverge, is combined.
As may be seen, none of these techniques uses only plastic material reinforced with fibres for producing the rim and/or the branches in the manufacture of a steering wheel particularly for a motor vehicle.